The present invention relates to a mold for making a magnetic tape cassette and to a method for molding a magnetic tape cassette or the like. Particularly, the invention relates to an improved mold and molding method for forming a magnetic tape cassette whose body is produced using a multi-color molding process.
In a conventional magnetic tape cassette for audio or video equipment or the like, a pair of hubs on which a magnetic tape is wound are rotatably supported in the cassette body. The body is composed of upper and lower halves, each of which is usually manufactured by the injection molding of a plastic or the like.
The cassette body is provided with one or more transparent or semitransparent windows through which the winding state of the magnetic tape can be viewed from the outside. Even if the entire cassette body is made of an optically transmissive material, the windows are often separately formed of a different optically transmissive material. This is due to the fact that the windows should have a higher optical transmissivity than the cassette body itself to provide for ease of viewing therethrough. The windows may be shaped in desired forms to produce a desired design effect.
Each of the windows is manufactured by bonding a separate window member to the cassette body through the use of an adhesive or using a process of ultrasonic welding or the like. However, there is a drawback in that it is necessary to separately manufacture the window member and thereafter to bond it to the cassette body. If the cassette body is made of the same optically transmissive material as the windows, there is another drawback in that the bonded portion of the cassette body is made more visible, thus generally degrading its appearance.
In order to eliminate such drawbacks, a method of manufacturing a magnetic tape cassette using a two-color molding process has recently been adopted. However, forming the window using the two-color molding method also creates problems. When one or more windows having a relatively complicated form are to be provided, as illustrated by windows 3a and 3b in FIG. 6, after the injection molding of the cassette body, cores having the same forms as the windows are moved in the direction of thickness of the body 2 of the cassette so that openings having the same forms as the windows are defined. A plastic for molding the windows is then injected in the direction of the thickness of the cassette body 2 through a direct gate or the like. At that time, slender intermediate portions already molded in the body, such as portion 2a located between the openings 3a and 3b, undergo either deformation due to the pressure or heat of the plastic injected into the openings or a color change due to resin streaking so that a desired form or appearance cannot be obtained.
These problems were successfully solved by the method described in the specification of commonly assigned Japanese Patent Application No. 62-16202. In the method disclosed in this application, which relates to the formation of a magnetic tape cassette by multi-color molding, an initially formed opening in a mold is filled with a molten resin that is to form the body of the cassette. Thereafter, a plurality of cores movable to change the size of said opening are moved to positions so as to define a new opening that communicates with a submarine gate, and the new opening is filled with a resin different from the previously charged resin through the submarine gate so as to mold the cassette body in more than one color.
This method, however, was later found to have several problems. When a submarine gate is used, it is generally desirable for the gate portion to be cut off the molded part (i.e., either half of the cassette) at a point where the resin temperature is fairly high. However, in the process of two-color molding described above, the body of the cassette is first molded before the windows are molded, so that by the time the molding cycle is completed, the sprue-runner to the cassette body through which the first resin was injected will have undergone excessive cooling, causing the resin to solidify to an unduly great extent. The resin then becomes extremely rigid and causes several problems in the step of cutting off the gate from the cassette body.
First, the sprue-runner to the submarine gate which has undergone a high degree of resin solidification is so rigid that the gate cannot be cut off the cassette body 2 without producing a jagged and indented surface such as would occur if the gate were merely torn off. This may be typified by a gate mark 60 that is created on the body 2 as shown in FIG. 7.
Secondly, in order to cut off the submarine gate, the mold is usually opened and the molded part and the sprue-runner are pushed almost simultaneously out of the cavity in one die part, thereby allowing the gate to be cut off along one edge of the die part. Thus, as shown schematically in FIG. 8, when the sprue-runner S.sub.0 is pushed outwardly with an ejector pin 50, the frictional drag of this part on the mold M causes the central portion of the sprue-runner S.sub.0 (where the sprue-runner is in contact with the ejector pin) to be separated first, and then the end of the sprue-runner (i.e., the gate portion G) is separated. In other words, the sprue-runner S.sub.0 is deformed in the shape of an arch and the gate is torn off by the pushing force of the ejector pin 50. Furthermore, the warped sprue-runner after being separated from the mold M rebounds violently in the opposite direction and vibrates for a certain period of time. In this case, the end of the sprue-runner S.sub.0 is subjected to a very strong centrifugal force and impact, and hence the chance of this portion (i.e., the cut face that has hardened excessively and which has become fairly brittle on account of cracking and other flaws that have developed as a result of tearing) of flying into pieces is increased. These pieces are not only dangerous to the operator, but they can also damage the demolded cassette half by producing a bruise or other surface flaws. The equipment around the injection molding machine can also be damaged by the flying pieces. Moreover, if such pieces are left on the parting line (PL) on the mold or within the mold cavity, not only is the efficiency of the molding operation reduced, but also the quality of the molded part will be adversely affected.
The severity of these problems associated with the flying of the end of the sprue-gate into pieces increases if cracking and other defects occur in the area at which the gate is cut off the molded part. In addition, if the number of molding steps is increased (i.e., more colors are used in the multi-color molding process), the cycle time becomes correspondingly longer, thus increasing the extent of deterioration in the cutting property of the gate.
The effect of such a poor cutting property of the gate portion is not limited to poor workability of molded parts. The flying pieces (fine particles) of the gate-forming resin tend to be deposited on the surface of the newly produced cassette half, thereby degrading the performance of the assembled tape cassette in that problems such as dropout become more pronounced.
It has been proposed that the submarine gate be cut off the molded part after reheating the sprue-runner. However, this method requires precise adjustments of the heating temperature; if the heating of the sprue-runner is excessive, not only is the cutting property of the gate impaired, but also the sprue-runner becomes too soft to be efficiently removed from the mold cavity automatically by suction or other suitable means. A further problem that occurs with this method is that the balance of cooling the molded part can be easily disturbed, causing deformation in the molded part.